cext: add Python 3.10 support * Replace "Py_TYPE(obj) = type;" with "Py_SET_TYPE(obj, type);" * Add pythoncapi_compat.h header file to get Py_SET_TYPE() on Python 2.7-3.8. Header file added to mercurial/ and contrib/python-zstandard/zstd/common/. In Python 3.10, Py_TYPE(obj) must not longer be used as an l-value. pythoncapi_compat.h comes from: https://github.com/pythoncapi/pythoncapi_compat Differential Revision: https://phab.mercurial-scm.org/D9825